1. Field of the Invention
The present invention relates to a composite sheet comprising layers of fibrous polyethylene. More particularly, the invention concerns such a sheet that is particularly useful as a substrate for printing.
2. Description of the Prior Art
Spunbonded fibrous sheet made of multiple plexifilamentary strands of oriented polyethylene film fibrils is known from, for example, Steuber, U.S. Pat. No. 3,169,899. Such sheet has been produced commercially by E. I. du Pont de Nemours and Company under the trademark "Tyvek" spunbonded olefin. The sheet has proven useful in many diverse applications, which take advantage of its unusually good combination of strength, tear and permeability properties, among others. However, in certain printing applications, improvement in the spunbonded fibrous polyethylene sheets are still desired. For example, in high density bar-code printing, the present inventors have found that the sheets sometimes exhibit inadequate print clarity. Accordingly, a purpose of the present invention is to improve the fibrous polyethylene sheet so that it performs satisfactorily in high density bar-code printing.
Even though the spunbonded fibrous polyethylene sheets are quite uniform, the present inventors found that the cause of the printing clarity problem was inadequate sheet-thickness uniformity.
Synthetic pulps of polyethylene are known in the art. Kirk-Othmer: Encyclopedia of Chemical Technology, volume 19, third edition, John Wiley & Sons, p. 420-435 (1982) describes synthetic pulps as generally being very fine, highly branched, discontinuous, water-dispersible fibers made of plastics. Known methods for producing the synthetic pulps include solution flash-spinning, emulsion flash-spinning, melt-extrusion/fibrillation and shear precipitation. The pulps may be blended with other fibers and made into papers, sheets or boards by conventional wet-lay papermaking techniques. Such pulps have also been used as bonding agents for certain nonwoven materials such as dry-laid, Rando-Webber formed sheets and wet-laid, Fourdrinier-formed sheets.
Gale et al, U.S. Pat. No. 4,608,089, discloses forming oriented polyethylene film-fibril pulps by cutting a flash-spun polyethylene sheet (e.g., Tyvek.sup..RTM.) into pieces, forming an aqueous slurry with the pieces and then refining the pieces with disc refiners to form a pulp that is particularly suited for cement reinforcement.
Composite nonwoven sheets also are known. For example, Weeks, U.S. Pat. No. 4,647,497, discloses a calendered composite nonwoven sheet comprising (a) a nonwoven scrim of continuous filaments of about 1 to 10 dtex per filament, preferably of polyester, polypropylene or nylon, (b) an abrasion-resistant synthetic pulp layer, preferably of polyethylene and (c) an adhesive binder which adheres the scrim to the pulp layer. The composite sheet is especially suited for air-infiltration barriers and outdoor signs and banners.